The structure of bioactive analogs of the N-terminal region of gastrin-17

Gastrin-17 (G17) processing intermediates bind to non-CCK receptors which mediate growth of the colonic mucosa but also the formation and development of colonic cancers. In previous studies, we removed the C-terminal region of G17 to form G17(1–12) and considerably shorter C-terminally amidated and non-amidated analogs. Peptides as short as G17(1–4) continued to bind to a single site on DLD-1 human colonic carcinoma cells, while only the G17(1–6)-NH2 and G17(1–12) peptides retained the ability to activate the receptor and stimulate cell proliferation in vitro. In this report, we studied the structure of these analogs, using a combination of ECD and VCD spectroscopy and replica exchange molecular dynamics (REMD) simulations in water, TFE, and membrane-mimicking environments, in order to determine preferred conformations that may have importance in promoting the biological activities. Mostly random meander structures, punctuated by a β-turn at residues 1–4, were found in most peptides by REMD simulations. G17(1–3)-NH2, which cannot form a β-turn, failed to bind the non-CCK receptor, suggesting the importance of this feature for binding. Additionally, the β-turn appeared more frequently in longer sequences, possibly explaining the higher affinity of the non-CCK receptor for these peptides seen previously. Finally, C-terminally amidated peptides generally showed greater formation of turn structure than their non-amidated counterparts as shown by ECD spectra, suggesting the importance of peptide length in stabilizing turn structure in N-terminal sequences, and perhaps explaining the ability of G17(1–6)-NH2 to activate the non-CCK receptor where as the non-amidated G17(1–6) and shorter peptides do not.